Mailpieces, in particular letters, in big quantities are in most cases automatically sealed and franked by the sender. For this purpose, amongst others, closing devices and franking machines are used, wherein both components are often connected with each other and sometimes form structural units. As an example, reference is made to the document DE 20 2004 011 390 U1.
In connection with the sealing of mailpieces, the sealing flaps of which are provided with a gumming, it is necessary to wet the gum before sealing. For this purpose, a sealing liquid is (automatically) applied to the gum by means of a wetting sponge. Then immediately after wetting the sealing flap is folded and pressed against an opposed surface of the mailpiece envelope, and thus the mailpiece is sealed.
From the practice, different sealing liquids are known. These are typically water or aqueous solutions, which on the one hand may contain biocides and fungicides, in order to prevent the occurrence of germ colonies and unpleasant odors. On the other hand, a wetting agent may be added.
Thus sealed mailpieces are normally opened by means of a letter opener or the like, i.e. the mailpiece is slit open in the area of an edge, and the contents are then taken out. However, a person, who is not authorized for opening, can open such a mailpiece for instance by watering it or by exposing it to a water vapor-containing atmosphere. Then the adhesive of the gumming will be dissolved in the water depositing in the area of the adhesion and the sealing flap can be lifted. The unauthorized person can then inspect the contents of the mailpiece and thereafter re-close the mailpiece with contents by folding and pressing-down the sealing flap. With careful operations, the authorized person later receiving the mailpiece cannot easily find out that before an unauthorized opening of the mailpiece has been performed. This is disturbing for obvious reasons.
From the document DE 4436294 A1 a sealing liquid is known in the art, which contains, besides the main component water, a second component of a two-component adhesive. The first component of the two-component adhesive is comprised in the gumming.
From the documents DE 3431239 A1 and DE-G 8716046, a safety letter envelope or a safety label is known in the art, wherein adhesive sections are disposed such that with (unauthorized) opening an irreparable destruction of the label or of the letter envelope will take place, which then can easily be identified.
From the document DE-OS 2319866, an enveloping machine with a wetting device is known in the art.